The present application claims the benefit of the filing date of U.S. Provisional Application No. 60/370,728, filed Apr. 9, 2002, and U.S. Provisional Application No. 60/310,844, filed Aug. 9, 2001, which are incorporated herein by reference in their entireties.
1. Field of the Invention
This invention relates to a novel process for producing a monoester mixture which possesses a desirable composition and color.
2. Background Art
In general, the preparation of propylene glycol fatty acid esters is possible from a number of routes. For example, propylene glycol and triglycerides can be reacted together to give a reaction product comprising monoesters of propylene glycol, propylene glycol diesters, monoglycerides, diglycerides, and triglycerides, after removal of the excess propylene glycol and glycerol. A second route is through the reaction of propylene glycol with fatty acids or fatty acid esters, such as methyl or ethyl esters of fatty acids. The product from this reaction will generally be a mixture comprising primarily mono- and diesters of propylene glycol after the removal of water or the low-boiling alcohol (ethanol, methanol, etc.), by-products and any excess starting reactants. A third route is to react propylene oxide with fatty acid, leading to a mixture of monoester isomers. A fourth route is to react propylene glycol with an acid chloride of a fatty acid.
Commercially, propylene glycol monoesters can be prepared by either directly esterifying propylene glycol with fatty acid or by interesterifying triglyceride with propylene glycol. Direct esterification under practical conditions can be accomplished by reacting propylene glycol with a fatty acid to yield approximately 55 to 60 percent of a propylene glycol monoester product; the balance is a reaction by-product comprising diester and unreacted starting material. Because of the high cost of fatty acids relative to triglycerides, the direct esterification process is not commonly utilized. The most commonly utilized process of making propylene glycol monoesters is by interesterifying triglycerides with propylene glycol. This interesterification reaction proceeds at temperatures ranging from 350xc2x0 to 450xc2x0 F. with the use of a catalyst such as sodium hydroxide. The resulting crude product contains propylene glycol mono- and diesters, monoglycerides and diglycerides, as well as numerous by-products. The final product composition of these processes can be described in terms of the ratio of mono- to diesters comprising the product. The composition of the end product can be controlled by varying the amounts of polyol with respect to oil, and through manipulating the reaction conditions. A higher concentration of monoesters is usually obtained through a molecular distillation process.
In many instances, it is preferable to utilize a monoester mixture which possesses a high ratio of monoester to diester. A typical commercial operation utilizes a high ratio of propylene glycol to fat to yield a product possessing a high monoester content usually 65% to 75% mono-ester. However, the crude monoester mixtures are typically purified for at least partial isolation of the monoesters from the diesters. Distillation or extraction of the crude monoester mixture produces purified monoester compositions (U.S. Pat. Nos. 3,669,848 and 6,153,773). In general, a distillation process is the most widely used technique for such purification. Typically the crude monoester mixture is distilled under vacuum, in a short path distillation process. The distillate generally comprises greater than 90% (by weight) monoesters. The remaining material generally comprises mainly diesters.
Monoglycerides are mono-fatty acid ester derivatives of the polyol, glycerol. In general, crude monoglyceride mixtures are made from reacting naturally occurring triglycerides, often obtained from oil seed processing, with glycerol. The process is known as glycerolysis. Such reactions generate a mixture of monoglycerides, diglycerides and triglycerides. Limitation on monoglyceride production, via this approach, is generally controlled by: (1) solubility of the glycerol in the reaction mixture; (2) the overall equilibria statistics; and, (3) time. Typical commercially available crude monoglyceride mixtures made using this approach include ratios of monoglyceride:diglyceride:triglyceride (by weight) of about 45:45:10; or about 60:35:5, depending on processing conditions used.
In many instances, it is preferred to utilize more purified monoglycerides. That is, crude monoglyceride compositions or mixtures are purified for at least partial isolation of the monoglycerides from the diglycerides and triglycerides. In general, monoglyceride distillation has been the most widely utilized technique for such purifications. Typically the crude monoglyceride mixture is distilled under vacuum, in a short path distillation process. The distillate generally comprises greater than 90% (by weight) monoglycerides. The remainder generally comprises diglycerides and triglycerides. During the process, the monoglycerides are generally heated to at least 200xc2x0 C. Sometimes processes which involve distillation of monoesters, such as monoglycerides or propylene glycol monoesters, are associated with the generation of xe2x80x9coff tastesxe2x80x9d and/or xe2x80x9coff aromasxe2x80x9d in the final product. The specific source of these off flavors or off aromas is not presently known. However, it seems to be associated with the conduct of distillation processes, i.e., processes that concern heating mixtures containing the monoglyceride (or propylene glycol monoester) of interest until they vaporize under the distillation conditions, typically 240xc2x0 C.
Typically, the esterification of polyols to produce monoester mixtures is catalyzed by strong base. For example, the most commonly used catalyst is NaOH (Hui Y. H., xe2x80x9cManufacturing Processes for Emulsifiersxe2x80x9d in Bailey""s Industrial Oil and Fat Products, John Wiley and Sons, Inc.(1996), 5th Ed., Vol. 4, pp. 569-601). However, there are reports of monoglyceride production in the absence of catalyst (U.S. Pat. Nos. 3,083,216; 2,197,339 and 2,197,340). These processes either required a distillation or did not produce a high monoester content of about 90%. Alternatively, an acid such as para-toluene sulfonic acid catalyzes the esterification of palmitic acid and propylene glycol (U.S. Pat. No. 3,669,848).
It is also desirable to prepare a monoester mixture of acceptable color. A dark-colored monoester mixture is not suitable for incorporation into products such as paint or food. A Lovibond tintometer is an instrument for evaluating colors on the Lovibond scale, which is a standard scale in the industry. The color of the monoester mixture can be determined by comparing the monoester mixture product to standard reference samples.
It would be useful to develop a process for producing monoesters from polyols and oils in which the product of said process contains a high amount (approximately 90%) of monoester and desirable characteristics such as light color without the need for purification through a distillation or extraction process. It has been found that the novel process disclosed herein yields a monoester mixture composed of about 90% monoester possessing acceptable color as measured by the Lovibond scale. Acceptable color is typically lower than 2 Red on the Lovibond scale.
The present invention is directed to a process of producing a high monoester mixture from a polyol and a triglyceride oil wherein said process eliminates the use of organic solvents, multiple water washings and/or molecular distillation. It has been found that in the presence of a catalyst, described herein, at a temperature range from about 180xc2x0 C. to about 280xc2x0 C. under an inert atmosphere or under the vapor pressure of the polyol with a pressure up to about 500 psig at reaction temperature, a polyol, described herein, interesterifies with an oil, described herein, to yield a monoester mixture possessing a desirable monoester composition and color. The present process yields a final product similar to that of processes which require the use of solvents, multiple water washings and/or molecular distillation.
In general, the present invention describes a process of preparing a monoester mixture which comprises:
1. A transesterification reaction in the presence of a triglyceride, a polyol and a catalyst, wherein the reaction mixture is heated from about 180xc2x0 to about 280xc2x0 C. in the absence of oxygen. It is important to remove oxygen from the reaction environment to prevent deleterious effects of oxidation products on the color of the reaction product. The reaction mixture comprising a triglyceride oil, a polyol and a catalyst is degassed by placing a sufficient vacuum on the reaction vessel to remove dissolved oxygen from the reaction mixture. Once the mixture is degassed and oxygen is removed from the headspace, the reaction proceeds under the vapor pressure of the polyol, and largely in the absence of oxygen. The amount of oxygen present is between about 0 and 160 Torr. Alternatively, oxygen is removed from the reaction vessel by a purge process, and the reaction may then proceed under an inert atmosphere, examples include N2, Ar and CO2 with a pressure of about 0 to about 500 psig. Typically, an amount of a triglyceride oil and a polyol is placed in a reactor vessel in the presence of a catalyst and an inert gas is bubbled through the liquid. The reactor headspace may be purged by evacuating the headspace under vacuum, followed by bubbling of an inert gas to fill the head space. The purge procedure is generally repeated three or four times. After purging, an inert atmosphere with a pressure of about 0 to about 500 psig is left above the liquid. Preferably the inert atmosphere is composed of an inert gas selected from N2, Ar, CO2 and the like. In all embodiments, it is most preferable that the inert gas is CO2. Preferably, the reaction temperature is between about 200xc2x0 and 260xc2x0 C., and the pressure is between about 30 and 400 psig. More preferably, the reaction temperature is between about 220xc2x0 and 240xc2x0 C., and the pressure is between about 100 and 400 psig. The oil is selected from the group consisting of: vegetable oils, fish oils, animal oils, and transgenically-modified plant oils, and derivatives including hydrogenated oils and mixtures thereof. The triglyceride oils may be saturated, unsaturated, or partially saturated. The vegetable oil may be a vegetable oil which contains conjugated fatty acids. The polyol is selected from dihydroxy polyols which include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol and the like. In a preferred embodiment, the oil is a vegetable oil, the polyol is propylene glycol (in any of its optical forms and/or mixtures) and the catalyst is a sodium or potassium salt of a mono- or di-carboxylic acid present in a concentration of between about 0.001% to about 10%. More preferably, the oil is soybean oil, linseed oil, corn oil, sunflower oil, canola oil, rapeseed oil, coconut oil, palm kernel oil, palm oil and hydrogenated derivatives thereof, the polyol is d,l-propylene glycol and the catalyst is a sodium or potassium salt of a monocarboxylic acid present in a concentration of between about 0.001% to about 10%. Most preferably, the oil is soybean oil, the polyol is d,l-propylene glycol and the catalyst is potassium acetate present in a concentration of between about 0.01% to about 1.0%. Preferably, the molar ratio of polyol to oil is about 8:1 to about 14:1. Most preferably, said molar ratio is about 12:1.
2. Neutralizing the reaction by adding a sufficient amount of acid to neutralize the catalyst. Preferably, the acid is phosphoric acid, e.g., 85% phosphoric acid.
3. Separating the excess propylene glycol. The term xe2x80x9cseparatingxe2x80x9d comprises useful processes known in the art for segregating components of a mixture as described herein. Such processes are decanting or centrifuging and the like. Typically, decanting comprises adding with mixing about 3% by wt. de-ionized water, allowing the organic and aqueous layers to separate and draining off the excess propylene glycol/glycerol/water layer.
4. Deodorizing the monoester mixture product in a manner known to those skilled in the art at a temperature from about 130xc2x0-140xc2x0 C. The residual propylene glycol and glycerol is removed as the deodorizer distillate (about 15-20% of the product weight).
5. Optionally treating the deodorized monoester mixture product with silica. Preferably, the deodorized product is contacted with about 1% of Trisyl 600 silica at a temperature of about 70xc2x0 C. to remove residual salts if necessary. Remaining moisture is removed under vacuum at a temperature of about 95xc2x0 C., and the silica is filtered from the product.
The product of the process of this invention possesses desired characteristics which include typically about 90% monoester content and a light color. Generally, the present process yields a monoester mixture product comprising about 90% monoester content and a Lovibond color of less than 2 Red (American Oil Chemists"" Society Official Method cC 13e-92 in Sampling and Analysis of Commercial Fats and Oils, pp. 1-3).
In another embodiment, a monoester mixture is produced by agitating a triglyceride oil, a polyol and catalyst to form a mixture, and subjecting the mixture to a vacuum. In this embodiment, a mechanical pump provides a reduced pressure atmosphere in the headspace of the reaction vessel above the mixture. The mixture is heated and agitated under a vacuum. Preferably, the mixture is heated to a temperature from about 5xc2x0 C. to about 120xc2x0 C., and the vacuum is from about 1 to about 30 inches of Hg. More preferably, the mixture is heated to a temperature from about 25xc2x0 C. to about 90xc2x0 C., and the vacuum is from about 10 to about 30 inches of Hg. The reaction vessel is then sealed by any means capable of closing the reaction vessel from the ambient atmosphere, said reaction vessel is capable of holding a range of pressures. An advantage of this process is that a high-pressure reaction vessel is not required.
The mixture agitating under a vacuum in the sealed reaction vessel is then heated to a temperature of between about 180xc2x0 C. to about 280xc2x0 C., wherein a vapor pressure due to the polyol is produced in the headspace of the reaction vessel. More preferably, the temperature is between about 200xc2x0 C. and 250xc2x0 C., most preferably about 240xc2x0 C. The pressure within the reactor is generally from about 0 to about 60 psig, depending on the temperature and the polyol of the mixture.
The triglyceride oil is selected from the group consisting of: vegetable oils, fish oils, animal oils, and transgenically-modified plant oils, and derivatives including hydrogenated oils and mixtures thereof. The triglyceride oils may be saturated (i.e., hydrogenated) or partially saturated. The triglyceride oil may be a vegetable oil which contains conjugated fatty acids. The polyol is selected from dihydroxy polyols which include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol and the like. In a preferred embodiment, the oil is a vegetable oil, the polyol is propylene glycol (in any of its optical forms and/or mixtures) and the catalyst is a sodium or potassium salt of a mono- or dicarboxylic acid present in a concentration of between about 0.001% to about 10%. More preferably, the oil is soybean oil, linseed oil, corn oil, sunflower oil, canola oil, rapeseed oil, and hydrogenated derivatives thereof, the polyol is d,l-propylene glycol and the catalyst is a sodium or potassium salt of a monocarboxylic acid present in a concentration of between about 0.001% to about 10%. Most preferably, the oil is soybean oil, the polyol is d,l-propylene glycol and the catalyst is potassium acetate present in a concentration of between about 0.01% to about 1.0%. Preferably, the molar ratio of polyol to oil is about 8:1 to about 14:1. Most preferably, said molar ratio is about 12:1.
The preceding process may further comprise the steps of:
i. cooling the mixture to a temperature from about 20xc2x0 C. to about 100xc2x0 C.,
ii. neutralizing the catalyst,
iii. collecting the monoester mixture produced from said mixture, and
iv. stripping the monoester mixture product at a temperature from about 50xc2x0 C. to about 150xc2x0 C., and a pressure from about 0 to about 10 millibars.
Neutralizing the reaction comprises adding a sufficient amount of acid to neutralize the catalyst (to a pH of about 6-7). Preferably, the acid is phosphoric acid, e.g., 85% phosphoric acid.
Collecting the monoester mixture comprises any known methods for separating a multi-phase mixture such as centrifuging and decanting. Decanting comprises allowing phases to separate and draining off the top layer to separate the layer containing the product. The layer containing the product can be stripped of residual polyol and polyol by-products such as glycerol and propylene glycol. As an example, the layer containing the product can be stripped on a wiped-film vacuum stripper at a temperature of about 60xc2x0 C. to about 100xc2x0 C., and a pressure of about 1 to 3 millibars (measured after the condenser) to remove glycerol and propylene glycol.
The stripped product is typically low in odor and has a Lovibond color of less than 2 Red.